1. Field of the Invention
The present invention relates to mixtures for blackening ceramic products by using aqueous solutions of ruthenium compounds.
2. Description of the Related Art
It has been disclosed that ruthenium chloride can be used in an aqueous solution for dyeing porcelain gray-black. During this process, a strongly acid aqueous solution of the ruthenium chloride is applied to the ceramic body and is burnt in; during this process, the chloride decomposes and the ruthenium oxide is left in superfine form and combines with the silicic acid or alumina of the ceramic bodies while forming color pigments (see Ullmann, Enzyklopadie der technischen Chemie [Encyclopedia of Technological Chemistry], 2nd edition, volume 4, pages 837 and 838).
The Czech patent application CS 248 541 B1 discloses that ruthenium oxide in zirconium silicate crystal lattices results in a gray-black color, so that such compounds can be used as black pigments.
Furthermore, EP 0 704 411 A1 describes that ruthenium salt or ruthenium complexes with polycarboxylic acids result in a dark gray coloration (identified as "black") if the application takes place on the ceramic body by first pretreating (activating) the ceramic body with water, then dying it with the ruthenium solution and subsequently aftertreating it with water. But this method also leads only to comparatively dark but not genuinely black colorations.
Therefore, the problem arose to find means with which ceramic surfaces can be dyed jet black in a simple and economical manner.